Above: Adjustable Ring Mode (ARM) fiber laser welding eliminates the problems commonly associated with applications like body-in-white (BIW) parts.

Advances in fiber laser technology improve aluminum welding

February 2020 - Efforts to improve automotive fuel efficiency for gas-powered vehicles, along with the rise of electric cars, are creating a need for lighter weight materials in passenger vehicle designs. Using aluminum for certain body components, in particular, has proved its value in reducing vehicle weight without compromising strength and durability.

There are challenges, however, when welding the aluminum alloys to make body-in-white (BIW) parts. A new type of fiber laser technology solves the most serious challenge—namely “hot cracking”—when welding 6XXX series aluminum alloys.

Induced stresses

Aluminum alloys of the 6XXX series are particularly attractive for BIW applications due to their favorable combination of mechanical characteristics and the fact that they can be heat treated. However, welding 6XXX series alloys has been problematic, especially due to hot cracking. This occurs because of stress induced during solidification due to aluminum’s inherently high thermal expansion coefficient and the difference in temper between the heated weld area and the base material. This is highly sensitive to the exact percentages of alloying metals (silicon and magnesium) used in the alloy.

The solution in the past has been to employ filler wire of a composition which, when introduced into the weld pool, changes the percentages of alloying materials enough to move the process away from the most crack sensitive region.

The use of filler wire comes with a cost (for the wire material itself), and added process complexity.  For example, it can be difficult to weld tight radii using filler wire; it can limit weld geometry (that is, it works best when you have an edge to push the wire against); and it complicates the mechanics of the process head. Eliminating the use of filler wire would be an advantage.

Gradual heat

In terms of the laser welding process itself, the issue with traditional fiber laser welding is that the small diameter of the focused beam deposits a large amount of heat into a very limited area.  As the beam moves along the weld seam, this causes rapid heating and cooling which exacerbates the tendency of the material to hot crack.

The solution is therefore to modify the spatial distribution of applied laser power to make the heating cycle more gradual. One way to accomplish this is by using “beam wobble.” That is, operators can rapidly scan the focused laser spot over a small area so it appears to the material to be much larger. But this has the side effects of increasing welding system cost and complexity and limiting weld speed.   

A recently developed technology called Adjustable Ring Mode (ARM) fiber laser offers a new solution. Here, the intensity profile of the focused laser spot is modified so that it departs significantly from the traditional, single-peaked Gaussian distribution. This consists of a central Gaussian distribution spot, surrounded by another concentric ring of laser light. This is produced using a specialized delivery fiber that has a traditional circular core surrounded by another, annular cross-section fiber core.

Developed by Coherent, the HighLight FL-ARM technology places the power in the center while the ring can be independently adjusted and even rapidly modulated on demand. This leads to a virtually unlimited number of possible combinations in terms of the power ratio of the inner to the outer beam. These can be broadly grouped into the configurations shown in Figure 1. Basic patterns can be varied to deliver a wide range of processing characteristics to optimally service a diverse set of applications.

Test results

Besides Coherent’s own internal work on aluminum welding using this technology, Kuka Systems North America, Sterling Heights, Michigan, undertook an independent study. Kuka’s engineers performed a series of overlap weld tests, comparing the results of a traditional fiber laser with the ARM laser at a variety of different power ratios between the center and ring beams. Both lasers were focused on the work surface to a beam diameter of approximately 660 µm.

A 1.3-mm-thick top sheet of 5XXX aluminum was welded to a 1.5mm thick bottom sheet of 6XXX aluminum with zero gap. A range of laser powers were used, which delivered either near or full penetration of both sheets. No filler wire or material was used.

In the case of the ARM laser, a HighYAG RLSK Remote welding optic was used for beam focusing, along with a travel speed of 4.5 m/min. (consistent with current production requirements). No beam oscillation was used for either laser.

Optimal results with the ARM were achieved with a total power of 3.3 kW, and a ring-to-center power ratio of 1.55:1 (that is, higher ring power than center power). A cross section of the full penetration weld is shown in Figure 2. This weld displayed no visible surface cracking, minimal porosity and no micro-cracking. All these problems were evident in the welds produced using the standard fiber laser, as shown in the comparative photos (Figures 3 to 5).

Although best results were achieved with higher total power in the ring beam than the center beam, the focused ring spot is much larger, and therefore its power density is lower. As the leading edge of the ring beam preheats the material, the center supplies sufficient power to perform the actual welding, and the trailing edge of the ring beam post-heats the material.

This sequence maintains the metal at a higher temperature for a longer period than a traditional single laser spot, and provides sufficient control of the heating/cooling rate to prevent cracking.

The ARM beam configuration demonstrates the capacity to weld formerly problematic aluminum alloys without hot cracking, and with a reduction in weld defects. By eliminating filler wire, as well as shielding gas, it is simpler to implement. Finally, this new method supports higher weld speeds than are possible with filler wire.  FFJ